Time-of-Flight (ToF), also known as Time-of-Arrival (ToA), is often used to measure the distance between two wireless devices. The distance R can be easily calculated as fly time t multiplied by the traveling speed of the signal, i.e., R=c*t. For a radio frequency signal, c is approximately 3×108 m/s.
This direct conversion between time and distance is the foundation of many ToF based location estimation technologies. If the ToF is known between a device to be localized (DTBL), referred as mobile device hereinafter, and multiple devices with known locations (referred as anchors, or reference nodes hereinafter), the distances between a mobile device and anchors can be computed and subsequently the mobile location is estimated using multi-lateration, or other techniques. The localization based on ToA has been widely used in many wireless localization systems.
Given that a mobile node, or device, is generally not time-synchronized to anchors in a given network, a technique called Two-Way TOA (TW-TOA) is commonly used to estimate the location of the mobile device. TW-TOA techniques may require signals to be transmitted and received by both the anchor and the mobile device. By doing so, the round trip fly time is measured and the distance is calculated using the round trip delay as R=c*T/2, where T is the round trip fly-time. Such an implementation using TW-TOA is bandwidth and energy inefficient because of the large number of transmissions needed for each localization operation. A system using TW-TOA for localization operations often has a significant capacity limit, i.e., the total number of nodes, or updates are very limited.
A more efficient technique for localization than TW-TOA is based on measuring the Time-Difference-of-Arrival (TDOA). TDOA estimates the differences in the distance from the mobile device to a plurality of different anchors. The differences in distance are calculated by measuring the difference of time when signals arrive at each receiver anchor, which subsequently determines the flight time difference. There are a number of methods to realize TDOA-based locationing, such as downlink TDOA (DL-TDOA), such as the TDOA system described in U.S. Patent Application Publication No. 2015/0156746, ‘Method and System for Estimating the Location of a Receiving Device,’ hereby incorporated by reference, uplink TDOA (UL-TDOA), such as the UL-TDOA system described in US Patent Application Publication No. 2015/0185309, ‘Method and System for Estimating the Location of a Transmitting Device In a Wireless Network,’ hereby incorporated by reference, and Beacon synchronized TDOA (BS-TDOA). The present disclosure describes techniques to improve the performance of the TDOA systems and can be applicable to all TDOA schemes.
A DL-TDOA system is illustrated in FIGS. 2(a) and 2(b). At the start of a positioning ranging process, an anchor 2210 can transmit a first request (REQ) packet 2208. One or more anchors 2202 can respond to the REQ packet 2208 by each transmitting response (RSP) packets 2209. A RSP packet 2209 may only be transmitted by an anchor 2202 after it receives a REQ packet 2208. A mobile device 2103 can receive both the REQ and the RSP packets 2208, 2209, and can thereby determine time differences of arrival and thereby estimate its own position.
A UL-TDOA system is illustrated in FIG. 7. The UL-TDOA system can include at least one mobile device 3101 and a plurality of anchors 3101. At the start of a localization process, a mobile device, M 3103, can transmit a REQ packet 3208. One of the anchors in range, for example anchor a0 3101 can responds to REQ packet 3208 by transmitting a RSP packet 3209. Other anchors in range, for example anchors a1-a3 3101, can each receive both REQ 3208 and RSP 3209 packets to determine the time differences of arrival. The location of the mobile device M 3103 can thus be estimated, as described in greater detail in U.S. Patent Application Publication No. 2015/0185309, ‘Method and System for Estimating the Location of a Transmitting Device In a Wireless Network.’
Another scheme using a hybrid TW-ToA and TDOA, referred to herein as Beacon Synchronized TDOA (BS-TDOA) scheme is illustrated in FIG. 8. The BS-TDOA system can include at least one mobile device M 4103 and a plurality of anchors 4101, 4104. At the start of the process, one of the anchors, for example a0 4104, can transmit a REQ packet 4208. The mobile device, for example M 4103 can, after receiving REQ packet 4208, transmit a response packet RSP 4209. RSP packet 4209 can be received by all anchors 4101. Anchors 4101 can determine the time differences between when each RSP packet 209 is received. The location of the mobile device 4103 can then be estimated using the arrival time difference, as described in greater detail in U.S. Pat. No. 8,259,699, ‘Method and system for target positioning and tracking in cooperative relay networks,’ hereby incorporated by reference in its entirety.
In all three TDOA schemes described above, two types of packets are transmitted, a request (REQ) packet, and one or more response (RSP) packets. A RSP packet is only transmitted by a device only after it receives a REQ packet. The difference lies in the devices used to transmit these packets and to measure the time difference of the arrivals.                In DL-TDOA, an REQ is sent by an anchor, and an RSP is sent by another anchor. The time differences of arrival are measured by a mobile.        In UL-TDOA, an REQ is sent by a mobile, and one, or more RSP packets are sent by anchors. The time differences of arrival are measured by anchors.        In BS-TDOA, an REQ is sent by an anchor, and an RSP is sent by a mobile. The time differences of arrival are measured by anchors.        
For current ToA and TDOA localization schemes, the accuracy of the location estimate will be affected by the presence of the non-line of sight (NLOS) measurements. In aforementioned cases, the NLOS measurements can introduce a time delay bias between anchor and mobile that is a factor that needs to be mitigated. Additionally, mobile nodes are often moving within an area covered by a system. As mobile devices are moving through a system, it is often not possible to avoid the occurrence of NLOS measurements, though the bias is not always consistent as the mobile device moves into a more favorable location, the bias will disappear. For TDOA systems, the NLOS bias between anchor nodes can additionally have the same effect on accuracy in location determinations. The bias between anchors, however, is persistent as anchors are fixed and thus do not change locations.
In all three cases described above, the time of flight between anchors is used to estimate the position of mobile devices. Accurate measurements of time of flight is necessary is for obtaining accurate position estimates for the mobile devices.
The bias caused by NLOS packet transmissions can severely degrade the accuracy of the position estimate of the mobile device. The bias between anchors can be especially harmful as the bias is always present for all the mobile devices. The bias can negatively impact position estimates of all individual mobile devices using the anchors not within the line of sight of each other.
To avoid the problems associated with non-line-of-sight bias, it is common practice to carefully choose only the anchor pairs that are within Line-of-Sight (LOS) to each other as TDOA pairs. This, however, can sometimes be difficult to realize, especially in complicated indoor environments. Even in systems where it is possible to carefully chose anchor pairs within LOS, it may require increasing the total number of anchors needed for the coverage, or may significantly reduce the overall network efficiency. LOS systems may also require time-consuming, manual pairing, which can indirectly increase the installation complexity and cost.
Accordingly, a need exists for systems and methods that allow for a reduction in non-line-of-sight signal transmission bias to enhance position estimates for mobile devices.